Polypropylenes are versatile thermoplastic molding materials with excellent properties, including high stiffness, heat resistance and transparency. On the other hand, the flexibility and impact resistance of polypropylenes are inadequate so that they are generally incorporated with soft rubber components. Although incorporation of soft rubber components compensates for the insufficient flexibility and impact resistance of polypropylenes, the resultant polypropylene compositions have lowered heat resistance. Further, such polypropylene compositions are required to have improved low-temperature heat-sealability.
Accordingly, there has been a demand for a polypropylene composition that has excellent flexibility and impact resistance as well as sufficient heat resistance and low-temperature heat-sealability.
Meanwhile, crystalline polypropylenes have excellent mechanical properties including tensile strength, stiffness, surface hardness and impact strength; optical properties including gloss and transparency; and food sanitation properties including nontoxicity and odorlessness. These properties provide wide applications particularly for food packaging purposes. However, single layer films consisting of the crystalline polypropylenes shrink at heat seal temperatures so that difficulties are caused in heat sealing such films. Therefore, the crystalline polypropylene films are generally combined with a heat-sealing layer that comprises a polymer such as a low-density polyethylene or a propylene/ethylene random copolymer.
The heat-sealing layers made from such polymers are required:
(1) to be heat-sealable at considerably lower temperatures than are the substrate films (crystalline polypropylene films);
(2) to have high heat-sealing strength of little deterioration with time;
(3) to have good adhesion to the substrate films;
(4) to be as transparent as or more transparent than the substrate films;
(5) to cause no blocking during storage;
(6) not to adhere to bag-making machines or jigs of filling and packaging machinery; and
(7) to have superior scratch resistance.
However, traditional heat-sealing materials do not satisfy all these properties. For example, low-density polyethylene films, although heat-sealable at low temperatures, have poor heat-sealing strength, bad adhesion to the substrate films and low transparency, and are also liable to adhere to packaging jigs.
Propylene/ethylene random copolymers can meet the above properties (2) to (7) but fail to satisfy the property (1). Therefore, the polypropylene composite films that include a heat-sealing layer comprising a propylene/ethylene random copolymer have a narrow range of heat-seal temperatures. Accordingly, heat sealing of these composite films by automatic packaging or bag-making machines requires strict control of the heat seal temperatures. Other materials proposed so far for the heat-sealing materials include blends of the propylene/ethylene random copolymers with ethylene/α-olefin copolymers. Such blends have improved low-temperature heat-sealability relative to the propylene/ethylene random copolymers, but their transparency is inferior.
The present applicant has found that a propylene/1-butene random copolymer which contains 55 to 85 wt % propylene and has a crystalline heat of fusion between 20 and 80 J/g as measured on a differential scanning calorimeter, is effectively used as a heat-sealing material because of its high transparency and excellent low-temperature heat-sealability. The present applicant has proposed a heat-sealing layer for polypropylene films that is formed from a composition which comprises the propylene/1-butene random copolymer in an amount of 50 wt % or more and an isotactic polypropylene (JP-A-S54-114887). The heat-sealing layer comprising the above composition has excellent low-temperature heat-sealability and blocking resistance, but is rather inferior in blocking and scratch resistances to the heat-sealing layers from the propylene/ethylene random copolymers. The present applicant has also proposed a composite film with excellent heat-sealability (JP-B-S61-42626); the composite film comprises an isotactic polypropylene film and a heat-sealing layer that comprises a composition containing the propylene/1-butene copolymer in an amount of 10 to 40 wt % and a crystalline propylene/α-olefin random copolymer.
Moreover, these polypropylene films need further improvements to meet the demand for higher-speed packaging. For example, excellent slip properties and blocking resistance as well as enhanced low-temperature heat-sealability are required.
JP-A-H08-238733 discloses a composite film that includes a heat-sealing layer comprising a metallocene-catalyzed propylene/1-butene copolymer and a crystalline propylene/α-olefin random copolymer. This reference has a problem that when the propylene/1-butene copolymer has a melting point of around 70° C., crystallization rate is lowered to cause bad productivity. Also, the moldability and the appearance of the film are more deteriorated when the propylene/1-butene copolymer has a large amount.
Metallocene compounds are of much interest recently as homogenous catalysts for olefin polymerization. Olefin polymerization with use of the metallocene compounds, particularly stereoregular polymerization of α-olefins, has been studied by many since the report of isotactic polymerization by W. Kaminsky, et al. (Angew. Chem. Int. Ed. Engl., 24, 507 (1985)).
In α-olefin polymerization using the metallocene compounds, it has been found that the stereoregularity and the molecular weights of resultant α-olefin polymers are widely varied by use of the compounds in which a substituent group is introduced into a cyclopentadienyl ring of a ligand or in which two cyclopentadienyl rings are bridged.
For example, propylene polymerization in the presence of a metallocene compound having a ligand in which a cyclopentadienyl ring and a fluorenyl ring are bridged, will yield stereoregular polymers such as:
syndiotactic polypropylenes when the polymerization is catalyzed by dimethylmethylene(cyclopentadienyl)(fluorenyl)zirconium dichloride (J. Am. Chem. Soc., 110, 6255 (1988));
hemiisotactic polypropylenes under catalysis by the above compound with introduction of a methyl group into the third position of the cyclopentadienyl ring, i.e. under catalysis by dimethylmethylene(3-methylcyclopentadienyl)(fluorenyl)zirconium dichloride (JP-A-H03-193796); and
isotactic polypropylenes under catalysis by the above compound with introduction of a tert-butyl group into the third position of the cyclopentadienyl ring, i.e. under catalysis by dimethylmethylene(3-tert-butylcyclopentadienyl)(fluorenyl)zirconium dichloride (JP-A-H06-122718). Further, dimethylmethylene(3-tert-butyl-5-methylcyclopentadienyl)(fluorenyl)zirconium dichloride can catalyze polymerization of propylene to provide higher isotacticity when tert-butyl groups are introduced into the third and sixth positions of the fluorenyl ring (i.e. dimethylmethylene(3-tert-butyl-5-methylcyclopentadienyl)(3,6-di-tert-butylfluorenyl)zirconium dichloride) (WO01/27124).
With respect to the influence on the molecular weights:
dimethylmethylene(cyclopentadienyl)(fluorenyl)zirconium dichloride can produce syndiotactic polypropylenes having higher molecular weights when the bridging group between the cyclopentadienyl ring and fluorenyl ring is altered to a diphenylmethylene group (i.e. diphenylmethylene(cyclopentadienyl)(fluorenyl)zirconium dichloride) (JP-A-H02-274703);
dimethylmethylene(3-(2-adamantyl)-cyclopentadienyl)(fluorenyl)zirconium dichloride can produce isotactic-hemiisotactic polypropylenes having higher molecular weights when the bridging group is altered to a diphenylmethylene group (i.e. diphenylmethylene(3-(2-adamantyl)-cyclopentadienyl)(fluorenyl)zirconium dichloride) (Organometallics, 21, 934 (2002)); and
dimethylmethylene(3-tert-butylcyclopentadienyl)(fluorenyl)zirconium dichloride can produce isotactic polypropylenes having higher molecular weights when a methyl group is introduced into the fifth position of the cyclopentadienyl ring (i.e. dimethylmethylene(3-tert-butyl-5-methylcyclopentadienyl)(fluorenyl)zirconium dichloride) (JP-A-2001-526730).
Contrary, polypropylenes with lower molecular weights result when substituent groups are introduced into two adjacent positions in the cyclopentadienyl ring of a catalyst component (JP-A-2001-526730 and JP-A-H10-226694); for example, dimethylmethylene(3-tert-butyl-2-methylcyclopentadienyl)(fluorenyl)zirconium dichloride and diphenylmethylene(3,4-dimethylcyclopentadienyl)(fluorenyl)zirconium dichloride can catalyze polymerization so as to give lower molecular weight polypropylenes relative to dimethylmethylene(3-tert-butyl-5-methylcyclopentadienyl)(fluorenyl)zirconium dichloride and diphenylmethylene(3-methylcyclopentadienyl)(fluorenyl)zirconium dichloride, respectively.
Meanwhile, synthesis of metallocene compounds that have a ligand in which a cyclopentadienyl group with substituent groups at two non-adjacent positions (e.g. third and fifth positions) and a fluorenyl group are bridged via an (alkyl)(aryl)methylene group or a diarylmethylene group, has been unsuccessful. This is attributed to the troublesome preparation of such ligand by the established method due to difficult reaction between a fluorene metal salt and a 6,6-diphenylfulvene derivative whose five-membered ring is substituted with such as an electron-donating hydrocarbon group. Furthermore, such selective introduction of substituent groups into two non-adjacent positions is difficult with the method disclosed in JP-A-H10-226694.
In general, the polymerization catalysts containing the metallocene compounds are required for further improvements in terms of polymerization activity, stereoregularity and molecular weight control. In particular, a polymerization catalyst that contains a metallocene compound as described in JP-A-H10-298221 can copolymerize ethylene and propylene while avoiding fouling, but the resultant copolymer has a remarkably lower molecular weight than a propylene homopolymer obtained with the catalyst.
Also, a polymerization catalyst that contains a metallocene compound as described in JP-A-H10-120733 copolymerizes ethylene and propylene to provide a higher molecular weight copolymer with no fouling. However, since this polymerization catalyst essentially requires a specific combination of an ionic compound and a metallocene compound, its versatility is rather limited.
As described above, olefin polymerization, for example copolymerization of ethylene and propylene, with these catalysts containing the metallocene compounds, has been almost unable to produce polymers having high molecular weights.
The present invention aims at solving the aforesaid problems. The present inventors have developed a novel transition metal compound useful as an olefin polymerization catalyst component that has a ligand in which a cyclopentadienyl ring with substituent groups at two non-adjacent positions and a fluorenyl ring are bridged via an aryl-substituted carbon atom, and also an olefin polymerization catalyst containing the transition metal compound. The present invention has been accomplished based on these findings.
Polyolefin resins, such as propylene block copolymers, have many applications including daily necessities, kitchenware, packaging films, home electric appliances, machine parts, electrical parts and automobile parts. These products and parts are mainly manufactured by injection molding due to high productivity. When resin compositions that contain propylene block copolymers are injection molded, circular ripples, called flow marks or tiger marks, occur on molded articles in the cross-flow direction. Noticeable flow marks on surfaces deteriorate the appearance of the molded articles so that they are concealed by painting or the like according to need. To cover up or obscure the flow marks on the molded articles obtained from the resin compositions containing propylene block copolymers, the resin compositions are injected into a high-temperature mold. However, this process requires a special mold and also the molding cycle is prolonged, causing productivity problems.
On the other hand, JP-A-H10-1573 discloses a composition comprising a metallocene-catalyzed propylene block copolymer and an α-olefin copolymer rubber. The metallocene-catalyzed propylene block copolymers have low crystallinity due to an approximate 1% of occurrence of 1,3-insertion or 2,1-insertion of propylene monomer. As a result, their melting points fall around 150° C., while propylene block copolymers prepared using titanium catalysts have melting points of 160° C. or several degrees higher. Further, the metallocene-catalyzed propylene block copolymers have lower tensile strength and flexural strength properties and stiffness than propylene block copolymers prepared with use of titanium catalysts. Therefore, practical use of the compositions comprising the metallocene-catalyzed propylene block copolymers and α-olefin copolymer rubbers has been unrealized due to their inferior mechanical strength properties to the compositions comprising titanium-catalyzed propylene block copolymers and α-olefin copolymer rubbers.